1. Field of the Invention
This invention relates to an automatic drive mechanism for controlling a travelling speed of a vehicle at a preset value, and relates more specifically to an electromagnetic clutch mechanism used therein.
2. DESCRIPTION OF THE PRIOR ART
There have heretofore been proposed various types of automatic drive mechanisms for vehicles. In one type of the automatic drive mechanisms proposed in the past, each of which uses an electromagnetic clutch, a cylindrical coil bobbin is fixed to a portion of electromagnetic yokes on the side of an input shaft which yokes are constituted by two portions and surround the coil bobbin. The coil bobbin is incorporated with the input shaft to be rotated with the latter. The energization of an electromagnetic coil on the coil bobbin is performed by the use of slip-ring type contacts comprising slip rings and brushes, and a frictionally engaging portion comprising pins and pin reception holes is disposed at the circumferential portions on the opposing surfaces of the input shaft side yoke portion and the output shaft side yoke portion, whereby, when the electromagnetic coil is energized, the input and output shafts engage each other through the pins and pin reception holes thereby to drive a throttle link of an engine by the output shaft.
In this type of automatic drive mechanism the coil bobbin is incorporated with the input shaft and rotates together with the latter, so that it becomes necessary to have slip-ring type contacts arranged along an entire circumference of the rotational portion. Additionally, the slip-ring type contacts must be kept electrically insulated from the other portions of the automatic drive mechanism. Furthermore, such a type of automatic drive mechanism has disadvantages such that the slip-ring type contacts may give rise to a failure in the operation of the automatic drive mechanism because of a contact failure occurring at the slip-ring type contacts and also that the construction of the automatic drive mechanism becomes complicated because the slip-ring type contacts have to be insulated electrically from the other portions of the automatic drive mechanism and arranged along an entire circumference.